Combination locks of the direct dial type are known to the art, and are useful because of the relative simplicity of their operation. Ordinary combination locks (not of the direct dial type) must be operated by turning the dial a specified number of turns in a first direction, say four turns to the right, before stopping at the first number (or letter) of the combination, then a series of turns one less in number in the opposite direction, e.g., three turns to the left, to the next number of the combination, followed by a decreasing sequence of turns in opposite directions to set the other numbers into the lock and thereby position the tumblers.
As a practical matter, the operation of such locks is undesirably complex because of the need to remember and execute properly not only the combination itself, but also the dialing program, including both the number and direction of turns required before stopping at each number of the combination. Because of this complexity, locks with combinations of more than four numbers are rarely used, and such locks are relatively slow to set and require careful attention. Increasing complexity of the program with longer combinations puts a practical limit on the number of theoretically possible combinations. Moreover, locks having only three or four tumblers are more readily attackable by radiographic means than locks having a larger number of tumblers.
Direct dial locks are easier to operate because they require no complex program of dial turning. They can be operated by turning the dial directly to each combination number, and do not require opposite rotations of the dial between successive numbers. Moreover, such locks do not require any predetermined number of turns, or indeed, any turns of the dial between succeeding numbers of the combinations.
When the dial of a direct dial lock has been turned to a given number, that number is set into the lock by axial movement of an operating shaft (which may be the dial shaft itself), for example by pushing the dial inwardly a short distance. Absent any requirement of complex dial turning, it is thus practicable to use combinations of large sets of numbers, up to seven or more, and corresponding numbers of tumblers. This provides great resistance to surreptitious attack.